The Line tool draws a straight line segment. Segments can be at any angle and easily forced to be horizontal, vertical, or at a 45° angle on the page.
Creating a Line Markup
Lines > Line. on the Tips:
Attaching a Note to a Line Markup
You can attach a note to nearly any markup that does not have a text box of its own. Simply double-click the markup to open the Note pop-up window. Notes attached to markups are functionally similar to the Note Tool.
Adding an Action to a Line Markup
You can add an action to nearly any markup. Simply right-click the markup and select Edit Action to open the Action dialog box.
Resizing or Adjusting a Line Markup
Select a markup to reveal its control handles. Each handle controls a different aspect of the markup's size and orientation.
To resize the markup or change its angle, click and drag the appropriate yellow handle. Hold down SHIFT while dragging to force the markup to a horizontal, vertical, or 45° angle.
See Editing Markups for more information.
Changing a Line Markup's Appearance
The appearance of the Line markup is configurable, including its color, thickness, pattern, ends, and opacity.
To change the appearance of a Line markup, select it and click the 
Properties tab.
Tabs > Properties or press ALT+P to show the Properties tab if it is hidden.
Appearance Properties
Color: Sets the line color.
Fill Color: Controls the fill color of the endpoints.
Highlight: Makes the fill color and line transparent so underlying content shows through.
Opacity: Sets opacity level of the line and endpoints, from 0 (invisible) to 100 (opaque).
Line Width: Sets the line thickness in points.
Style: Determines the pattern of the line segment.
Start and End: Controls the appearance of the endpoints for each side of the line. The diagram below shows samples of each type of endpoint.
Each endpoint has an associated Scale that determines the size of the endpoint in relation to the Line Width. By default, it is set to Auto and will automatically adjust as the Line Width is changed.